MAXIMUM BOUNDS ON THE ERA OF MARTIAN VALLEY NETWORK FORMATION

Valley networks offer compelling evidence for extensive liquid water on the surface of early Mars. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on a fluvial intermittency factor that varies widely for Earth’s rivers and is unknown for Mars. Here, superposition relationships are used to identify crater populations that pre- and post-date valley network incision, resulting in maximum valley formation times of ~10⁸ years. Although significantly longer than the 10⁵-10⁷ year valley formation timescales found using paleohydrologic reconstructions and the aforementioned intermittency, such long timescales are supported by hydrologic modeling of couple lake systems, large impact craters that contemporaneously disrupted valley flow paths, and the observations that most valley systems are poorly topographically integrated into the martian landscape.

Furthermore, assuming sediment transport rates previously calculated for other martian valley networks, the calculated timescales imply a minimum fluvial intermittency of ~10^-5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits, including the Jezero crater delta. These timescales correspond with long-term erosion rates of ~0.5 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity.